1. Field of the Invention
The invention relates to the testing of electrically programmable memory (EPROM and EEPROM) cells.
Such cells are each formed by a floating gate transistor, the control gate of which is connected to a word line, the drain to a bit line and the source to a reference potential that is generally the electrical ground of the circuit.
The cells are placed in a matrix arrangement: a word line is connected to the control gates of all the transistors of one line. A bit line is connected to the drain of all the transistors of one column.
To facilitate the addressing operation, the memory array is generally divided, for example to form sub-groups of bit lines. The column decoding (of the bit lines) is then done in two phases. These are: the decoding of a bit line in each sub-group and the decoding of the sub-group. This set of sub-groups forms a group that corresponds to a data element or again to a data bit. For this group, there will be only one reading amplifier. There could be one writing amplifier per sub-group, this amplifier being selected by the decoding of the sub-group.
In practice, the memories are organized in information words of a certain length: 4, 8, 16, 32 data bits. We then have several groups of bit lines as defined here above. A group of bit lines corresponds to the bits having a place value i in the information word. There are many groups as there are bits in the information word.
Furthermore, it is sought to optimize the occupancy of the surface of the semiconductor circuit. Thus, for example, for an eight-bit memory, namely a memory in which an information word is given on eight bits with place values 0, 1 , . . . , 7 corresponding to input/output pins of data D0, D1 , . . . , D7 of the memory, eight groups of bit lines are defined: a first group corresponding to the bits with the place value 0 (D0) , . . . , an eighth group corresponding to the bits with the place value 7 (D7). And, in this example, there are eight reading amplifiers, one per group.
It is also possible to define, for example, two groups of word lines: a first group corresponding to the groups 0, 1, 6 and 7 of bit lines D0, D1, D6 and D7, a second group corresponding to the groups 2, 3, 4 and 5 of bit lines D2, D3, D4 and D5. Such a choice depends on the physical structure chosen for the circuit and on its optimization.
The reading of a memory word will lead to the selection of a word line in each group of word lines and to the selection of a bit line in each group of bit lines. All the data bits of the information word are then given in parallel by the reading amplifiers associated with the groups of bit lines.
Topographically, as can be seen in FIG.1, a division such as this enables an organization in two half-arrays, the bit lines being taken vertically in the example and the word lines being taken horizontally. The upper half-array corresponds to the first group of word lines with the groups 0, 1, 6 and 7 of bit lines. The lower half-array corresponds to the second group of word lines with the groups 2, 3, 4 and 5 of bit lines. In the example, the second group of words corresponds to a simple placing of the first group in parallel. The pre-decoder (PREDEC) is preferably just above and on the border of the upper half-array. The word line decoder (DEC L) of each half-array is placed on the border of the vertical sides of the corresponding half-array. The column decoders (DEC C), namely the decoders of the bit lines, of the upper half-array, and the corresponding reading and writing amplifiers (AMPLI) are located between the two half-arrays. Those of the lower half-map are located beneath this half-array, on the border.
On the periphery of the memory circuit, there are the memory input/output and memory control pins with the associated buffers. Thus, at the top and on the vertical sides, there are input pins of the address bus, in the example A0-A15 with the associated input buffers. They are thus close to the different decoders. There are also the pins for ground (GND) and the Vcc and Vpp supplies and for the selection of the memory circuit NCE.
At the bottom, and somewhat to the right, there are the data (D0-D7) input/output pins, and the ground of the circuit GND. In the example, D0 is to the far left, D5 to the far right and D6 and D7 at the bottom of the straight vertical side. The interconnections of the upper half-map with the corresponding input/output pins D0, D1, D6 and D7 are thus optimized.
The interconnections to be made are numerous, between the decoders, the pre-decoder and the input buffers of the address bus, the amplifiers and the input/output buffers of the data bus. Thus, if the memory array proper is called the "core", the decoders and amplifiers that give access to the memory array are called the "middle" and the buffers and the input/output pins of the circuit are called the "periphery", then there are numerous interconnections to be made between the middle and the periphery, it being furthermore known that these interconnections cannot go through the "core". These interconnections take up a great deal of space with respect to the core, which it is why it is always sought to reduce their number. It is also sought to position the different circuits so as to optimise the length of the interconnections.
2. Description of the Prior Art
Now, for the requirements of testing electrically programmable memory cells, for which it is sought to read not the binary content 0 or 1 depending on whether they are programmed or not but rather their capacity to conduct current before or after their programming, additional connections are used to enable the measurement of the current that passes into the cell, when access is obtained to it in reading mode. The testing circuit generally used is shown in FIG. 2, where core, middle and peripheral portions of the chip are indicated as C, M, and P, respectively.
Indeed, in reading mode, the drain of the selected cell C1, namely in fact the corresponding bit line LB, is connected to the input of a reading amplifier AL which will inject current into the cell. Depending on whether the current goes into the cell or not, the amplifier delivers a high or low logic level at its output. A connection DX connects the output of the reading amplifier to the input of the corresponding input/output buffer (I/O) which is itself connected at output to a corresponding input/output pin, for example D2.
In testing, the cell also is read, but it is sought to read the current that goes through it and not the binary logic level delivered at output of the reading amplifier. An additional connection DIX is then used. This additional connection DIX connects the input/output pin D2 to the drain of the cell C1 by a transistor T used as a switch (and gated by signal DMA described below), its source being connected to the connection DIX and its drain through node A to the drain of the cell. This transistor is topographically beside the amplifier, namely in the middle zone. When the circuit is in testing mode, a signal DMA puts the input of the reading amplifier and the output of the input/output buffer in a high impedance state, and makes the transistor/switch T conductive. Thus, if an external voltage is imposed on the pin D2, it is possible to read the current that flows into the cell. This current is disturbed neither by the input/output buffer nor by the amplifier since these two elements have been placed in a state of high impedance.
Having a test circuit such as this amounts to doubling the interconnections DX: the number of interconnections between the reading amplifier and the input/output buffer is doubled. This is a major drawback as has already been seen in the optimization of the size of the circuit, since the number of connections directly and substantially affects the size of the circuit.